The present invention relates to stable, concentrated dispersions of nanometric silver particles, and to a method of producing such dispersions.
Nanometric silver particles are seeing increasing use in the pharmaceutical industry, especially in the field of wound management. Dispersions of fine silver particles are widely used in the manufacture of conductive inks and electrically conductive films for applications such as internal electrodes in multi-layer capacitors, interconnections in multi-chip components, conductive lines in defoggers and deicers, photovoltaic modules, resistors, inductors, antennas, membrane switches, electromagnetic shielding, thermally conductive films, light reflecting films, and conducting adhesives. In many existing and emerging technologies, demand for ultrafine silver particles having specific properties is increasing.
In some applications, required features of such particles may relate to at least one of: average particle size, narrow particle size distribution, particle density, and crystalline grain structure.
Nanometric silver particles have seen commercial use as dispersions in organic solvents. The stability of such dispersions may typically be guaranteed for up to six months.
The production of fine silver particles by precipitation of the particles from a liquid medium is well known. However, as taught by U.S. Pat. No. 6,277,169 to Hampden-Smith, et al., such liquid precipitation techniques are often difficult to control to produce particles having the desired characteristics. Specifically, U.S. Pat. No. 6,277,169 discloses that it is especially difficult by the liquid precipitation route to obtain particles having dense, spherical particle morphology and good crystallinity.
The production of small silver particles by reducing silver oxide (e.g., using hydrogen peroxide) in aqueous liquid media is known. The sparingly soluble silver oxide may be dissolved in the reaction media prior to the reduction reaction by complexing the silver ions with ammonia, as disclosed by WO Patent Publication No. 2003/080231.
Various liquid precipitation techniques may promote agglomeration or aggregation of the silver particles, as well as particle sedimentation. Such agglomeration may be undesirable for many reasons, among them: the particle size distribution and mean particle size may be deleteriously affected, and contaminants in the mother liquor may be occluded between particles, reducing product purity. We have found that the specific electric resistivity of thin films formed from such contaminated silver particles may disadvantageously be increased. Also agglomerated particles and/or sedimentation may clog the nozzles in ink jet printing heads, detracting from the robustness of the printing process.
Various liquid precipitation techniques may promote the formation of polycrystalline silver particles. We have found that such polycrystalline silver particles may disadvantageously exhibit a higher specific electric resistivity. Moreover, they may display an appreciably higher propensity to agglomeration and to sedimentation.
The advances in the production of silver nanoparticles notwithstanding, the present inventors have recognized a need for improved silver nanoparticles and silver nanoparticle dispersions, and for methods of producing such nanoparticles and dispersions thereof.